The invention relates to a bifocal contact lens of the bivisual type and, more particularly, without any image jump.
Bifocal contact lenses of alternating and bivisual types are available for presbyopic persons. The lenses of the alternating type include variously-configured, segmentally-ground (bifocal) lenses having an essentially-horizontal zone of separation or transitional line (or, more precisely, an area as used hereinafter) between two upper and lower areas of different vertex powers for the distance zone and the reading zone. Using this type of contact lenses, it is difficult to determine the correct segment height when fitting the lens. Furthermore, there is a considerable image jump at the transitional area in some lenses of this type.
In the known bifocal contact lenses of the bivisual type, which sometimes are also known as the simultaneous type, the distance-focusing part is in the center of the lens, and the reading-focusing part is located peripherally around the distance-focusing part. For bifocal operation, the distance part and the reading part must simultaneously cover portions of the pupil area, preferably one-half of the pupil area each, so that simultaneous imaging from the distance and reading parts takes place. This type of contact lens makes it possible to prescribe thin minilenses with relatively steep base curves, so that corneal temperature is kept low and corneal gas exchange is impaired as little as possible. Also, in cases of large pupil diameters of the lens wearers, the lens can be made large so that head attitudes are unnecessary either for reading or viewing at distances. However, double images, particularly of bright objects, and thereby reduction of image contrast are disturbing. Rotational movement of these concentric bivisual lenses is clearly not disadvantageous, so that additional means of stabilizing the position of the lenses on corneas are not necessary, but problems do occur with vertical movement of the lenses on account of differences in the intrinsic brightness (tunnel effect) of the images from the concentric, differently-focusing parts.
A review of various bifocal lens systems and the limits of their application is contained in a paper presented at the 14th Convention of the VDC in Nuernberg in 1969 by Guenter Giefer, entitled, "Die verschiedenen Bifokallinsensysteme - Moeglichkeiten und Grenzen ihrer Anpassung" ["bifocal lens systems - possibilities and limits of their application"] (printed in NOJ, June 1969) and in the textbook Kontaklinsen [Contact Lenses] by Heinz Baron, 1981, Verlag Optische Fachveroeffentlichung GmbH, Heidelberg.